The Easter egg was found by a Reddit user, and it comes from the tenth episode in the series. In that episode Misty Knight is digging into Luke Cage’s past and comes across a newspaper. In the right-hand column there’s an article that talks about a scientist who is clearly Doc Brown being commended at some benefactor’s ball, It mentions his Flux Capacitor and the possibility of making time travel possible when plutonium is available on every corner drugstore. You can read the whole thing in the image below.
Due to copyright issues, they changed a few things. For example they Flux Capacitor is called the “Thrust Capacitor,” they changed Marty McFly’s name to “Mac Fly,” and instead of being from Hill Valley he’s from “Mill Valley.”